Field of the Invention
The present invention relates to a robot system in which a visual sensor, such as a camera, measures a position of a work to correct an operating position of a robot body and to a calibration method of the robot system for calibrating the robot system.
Description of the Related Art
In recent years, automatic assembly by a robot is often used in a production line of a factory to automate operation and save manpower. In the automatic assembly, a system with a combination of a robot body, such as a multi-jointed arm, and a visual sensor, such as a camera, is used. Particularly, there is an increased need recently for a visual sensor to three-dimensionally measure a work as an object of operation.
When a visual sensor measures a position of a work to correct operation of a robot body based on the position, position and orientation data of the work measured by the visual sensor needs to be converted to data on a coordinate system fixed to the robot body. Therefore, a relationship between a coordinate system used for measurement data of the visual sensor (hereinafter, “vision coordinate system”) and a coordinate system fixed to the robot body (hereinafter, “robot coordinate system”) needs to be obtained in advance. Calibration between the coordinate systems is generally called “hand-eye calibration”. Calibration accuracy of a relative position and orientation between the vision coordinate system and the robot coordinate system is directly related to operating position correction accuracy of the robot body using the visual sensor. Therefore, it is desirable to increase the accuracy of the hand-eye calibration.
Conventionally, an example of a method of hand-eye calibration without using accurate design data or a special jig includes the following technique related to a robot arm including an on-hand camera on the wrist. More specifically, sensor output indicating positions related to the same object as a reference is obtained from at least three positions not arranged in a straight line. A method is proposed, wherein hand-eye calibration is performed by executing software processing based on data expressing the output on the robot coordinate system and data expressing the sensor output (see Japanese Patent No. 3644991).
Also proposed is an automatic hand-eye calibration system using a jig that indicates a positional relationship between a marker that can be observed by a sensor and a point for the robot to touch up (see Japanese Patent No. 2700965).
The relative positional relationship between the robot coordinate system and the vision coordinate system is calculated by these methods.
However, in the method described in Japanese Patent No. 3644991, the measurement accuracy of the camera may not be sufficiently utilized depending on teaching points used in the hand-eye calibration. In general, a measurement error of a camera is large in the optical axis direction due to influence of a quantization error. More specifically, when a camera including light receiving units with finite pixels measures the position of an object, detected positions of the object are discrete, and there is a quantization error.
A triangulation method is used to calculate the position of the object in the optical axis direction of the camera based on the distance between two points on the object in the horizontal axis direction of the camera. A position measurement error in the optical axis direction of the camera is composed of a position measurement error of two points in the horizontal axis direction of the camera, and the position measurement error in the optical axis direction of the camera is generally greater than the position measurement error in the horizontal axis direction of the camera. Therefore, when the relative positions and orientations of the object as a reference and the camera, that is, the teaching points used in the calibration, are searched by manual trial and error in the actual operation site, the teaching points suitable for the calibration are not selected in some cases, and there is a problem that the calibration accuracy is not sufficient.
In the method described in Japanese Patent No. 2700965, although a plurality of points are not necessary for the relative position and orientation of the marker as a reference and the camera, a special jig in which accurate design data is apparent is required. The cost of the special jig in which accurate design data is apparent is higher than a normal jig, and many man-hours are necessary.
In view of the circumstances, an object of the present invention is to provide a robot system and a calibration method of the robot system, wherein a special jig in which accurate design data is apparent is not necessary, and accuracy of calibration between a robot coordinate system and a vision coordinate system can be further improved.